Bases Potassium hydroxide-18-Crown

Cyclic polyethylene oxides) ( Crown ethers ), Potassium hydroxide Le Goaller, R. etal., Synth. Comm., 1982, 12, 1163-1169 Crown ethers promote dihalocarbene formation from chloroform or bromoform and potassium hydroxide. However, in absence of diluent dichloromethane, dropwise addition of bromoform to the base in cyclohexane led to explosions. 

In contrast, liquidiliquid phase-transfer catalysis is virtually ineffective for the conversion of a-bromoacetamides into aziridones (a-lactams). Maximum yields of only 17-23% have been reported [31, 32], using tetra-n-butylammonium hydrogen sulphate or benzyltriethylammonium bromide over a reaction time of 4-6 days. It is significant that a solidiliquid two-phase system, using solid potassium hydroxide in the presence of 18-crown-6 produces the aziridones in 50-94% yield [33], but there are no reports of the corresponding quaternary ammonium ion catalysed reaction. Under the liquidiliquid two-phase conditions, the major product of the reaction is the piperazine-2,5-dione, resulting from dimerization of the bromoacetamide [34, 38]. However, only moderate yields are isolated and a polymer-supported catalyst appears to provide the best results [34, 38], Significant side reactions result from nucleophilic displacement by the aqueous base to produce hydroxyamides and ethers. 

Compound 8 can be reacted with substituted iodoarenes using copper metal and potassium carbonate <1980CB358> or potassium hydroxide <1994MCL(242)127> as a base. The reaction proceeds at 170-180°C and gives good yields. Addition of crown ether 18-Cr-6 allows a lower temperature to be used <2000JCD2105>. 

Crown ethers promote dihalocarbene formation from chloroform or bromoform and potassium hydroxide. However, in absence of diluent dichloromethane, dropwise addition of bromoform to the base in cyclohexane led to explosions. 

Crown ether catalysis of the reduction is also useful (Table I), with potassium hydroxide as the base and 18-crown-6 as the catalyst (18). 

Generation of chloro(2-furyl)carbene and chloro(2-thienyl)carbene from 2-dichloromethyl-furans 1 or 2-diehloromethylthiophenes 2 and a base, followed by addition to an alkene, gave l-chloro-l-(2-furyl)- or l-(2-thienyl)cyclopropanes, respectively "(for earlier work, see Houben-Weyl, Vol. E19b, p 1001). The reactions were carried out using potassium /crt-butoxide as the base with, or without, 18-crown-6, or solid potassium hydroxide and a catalytic amount of benzyltriethylammonium chloride (TEBAC) (solid-liquid variant of a phase-transfer catalytic system ). Under both conditions, yields of cyclopropanes were usually good, yet the phase-transfer catalytic system seems to be more convenient from a practical point of view. Addition of the above described carbenes to alkenes was stereospecific. 

Among the many base-solvent systems applied for this purpose, phase-transfer catalysis is the most efficient and the simplest. Concentrated aqueous solutions of sodium or potassium hydroxide and a quaternary ammonium salt or a crown ether, e.g. dicyclohexane-18-crown-6, as a catalyst have been used. 

The reaction of chloro(methoxy)methyl tolyl sulfone with a base generates methoxy(4-tosyl)-carbene (carbenoid) which adds to 1,2-dimethoxyethene, straight chain and cyclic enol ethers or arylethenes to give 1-methoxy-l-tosylcyclopropanes. Concentrated (50%) aqueous potassium hydroxide with a catalytic quantity of 18-crown-6 or, in one case, butyllithium, was used as the base (see Houben-Weyl, Vol. El9b, pp 1730-1734). 

Many other examples of the formation of aza-crowns by this method were provided in the cited publications. The best solvents were dioxane and diglyme at 20-80°C. Tosyl chloride or benzenesulfonyl chloride was used with sodium or potassium hydroxide as the base. 

Chlorofluorocarbene. Schlosser et al. have found that this crown ether is significantly superior to the commonly used ammonium salts for generation of chlorofluorocarbene from dichlorofluoromethane by the two-phase technique. They used potassium hydroxide as base rather than sodium hydroxide, since crown ethers bind potassium ions more selectively than sodium ions. They used the carbene for synthesis of fluorodienes. The method is illustrated for the conversion of methallyl chloride (1) into 2-fluoro-3-methyl-l, 3-butadiene (3-fluoro-isoprene), (4). The conversion of (3) into (4) involves a 1,4-elimination of ICl ... 

EXPLOSION and FIRE CONCERNS nonflammable NFPA rating (not rated) explosive reaction with crown ethers or potassium hydroxide violent reaction with lithium, sodium-potassium alloy, acetone, or bases incompatible with metals, caustic alkali, and strong oxidants decomposition emits highly toxic gases and vapors (such as hydrogen bromide and bromine) use dry chemical, carbon dioxide, water spray, fog or foam for firefighting purposes. 

Propose a synthesis for 18-crown-6. If a base is used in your synthesis, does it make a difference whether it is lithium hydroxide or potassium hydroxide Explain. 

More efficient separations are obtained with the above-mentioned tetradecyl-substituted materials based on 18-crown-6. Ion-exchange interactions are dominating the separation mechanism in this case [46]. By using two of those stationary phases in series, 14 different inorganic and organic anions can be separated utilizing potassium hydroxide and acetonitrile as mobile phase (Fig. 3-59). The use of two stationary phases in series is necessary to ensure adequate capacity for the sufficient separation of all analytes. In comparison with conventional anion exchangers, crown ether phases exhibit less selectivity for divalent anions ... 

---